In the design of narrow band FM receivers for VHF and UHF signals, it would be desirable to use a superheterodyne circuit with a single intermediate frequency stage. However, this results in an intermediate frequency in the high frequency spectrum, such as 10 to 60 megahertz (Mhz). At such high intermediate frequencies, it has been expensive to build coil and condenser filters which will reject stations on adjoining channels. Further, at such high intermediate frequencies, conventional discriminator circuits for converting the frequency modulation into an audio output signal produce very low audio voltages and exhibit an unacceptable drift in its tuning adjustment. Heretofore, these problems have been averted by an FM receiver design with double or triple frequency conversion so that the final intermediate frequency is reduced to a few hundred kilohertz (Khz) where the desired selectivity and discriminator output and stability can be realized. Such a receiver design is relatively complicated and costly.
It is therefore desirable, to provide a narrow band FM receiver for the VHF and UHF bands which has a high intermediate frequency such as that which may be produced by a single frequency conversion. The desired selectivity at the high intermediate frequency can be provided by the use of low cost piezoelectric filters. There has remained however a problem of providing an FM discriminator which will provide a high level audio output and good tuning adjustment stability in a simple low cost circuit.
In the prior art, there are several well known types of FM and PM discriminators which used inductance-capacitance or tuned transformers to generate a quadrature voltage which is 90.degree. out of phase with the signal voltage at the center frequency and which is shifted in phase as the signal deviates from the center frequency. In the Foster-Seeley discriminator and in the ratio detector, the quadrature voltage is generated by a discriminator transformer.
Also in the prior art, it has been proposed to utilize a piezoelectric crystal in FM and PM discriminator circuits to obtain an audio output voltage having an amplitude which varies with frequency deviation of the modulated wave. In the Crosby U.S. Pat. No. 2,156,376, a crystal and a neutralizing capacitor are used in a discriminator circuit which develops opposed voltages which are combined to produce the discriminator output. Similar arrangements are shown in the Crosby U.S. Pat. Nos. 2,158,276; 2,397,840; 2,397,841; and 2,425,924. The Dix U.S. Pat. No. 3,160,822 discloses a narrow band FM discriminator which uses the voltage developed across a crystal at a point off resonance. The Nagata et al. U.S. Pat. No. 4,013,970 discloses a similar arrangement. Th discriminator circuits of these patents include a piezoelectric crystal in a circuit which utilizes voltage amplitude variations across the crystal as a function of frequency of the input signal voltage.
A general object of this invention is to provide an improved discriminator for FM and PM waves which overcomes certain disadvantages of the prior art.